Prostate cancer (PCa) is the second leading cause of cancer death in American men. Castration Resistance Prostate Cancer (CRPC) is the major cause of high mortality of PCa and is currently incurable. Though androgen receptor splice variants, such as AR-V7, have been implicated in CRPC pathogenesis and drug resistance, the underlining mechanisms remain poorly understood. This greatly impairs development of novel biomarkers and therapeutic targets to manage and treat this deadly disease. Our laboratory recently discovered that ARD1, an acetyltransferase, is an activator of AR-V7. We have shown that ARD1 is overexpressed in CRPC and it acetylates AR-V7 at lysine 618 (K618) to promote AR-V7 nuclear translocation. To further determine if the ARD1-AR-V7 axis can be used as a biomarker or therapeutic target for CRPC, we have generated a pair of CRPC cell lines - LNCaP95 cells carrying wild-type AR-V7 (AR-V7-ACWT) or carrying an acetylation abolished mutant AR-V7-618R (AR-V7-ACNULL) by using CRISPR technology. Moreover, we have collected 150 blood samples from metastatic CRPC (mCRPC) patients before or after treatment with abiraterone or enzalutamide and 11 from non-CRPC metastatic PCa (mPCa) patients. We have shown, in a small cohort, that ARD1 levels are higher in Circulating Tumor Cells (CTCs) from mCRPC patients than those from mPCa patients. These novel preliminary data and generated reagents allow us to hypothesize that ARD1 up-regulation favors CRPC development by acetylating AR-V7 at K618, which promotes AR-V7 dimerization and nuclear translocation, and therefore, inhibition of ARD1-AR-V7 axis inhibits CRPC pathogenesis and drug resistance. Therefore, we propose the following two Specific Aims: 1. Determine the roles and mechanisms of ARD1-acetylated AR-V7 in CRPC pathogenesis. We will first determine if ARD1- acetylated AR-V7 promotes CRPC pathogenesis by comparing the rates of cell proliferation, colony formation, and xenograft tumor growth between the paired cell lines of AR-V7-ACWT and AR-V7-ACNULL. Then, we will determine if K618 acetylation is required for AR-V7 homo- or hetero-dimerization with AR by using bimolecular fluorescence complementation (BiFC) assays since AR-V7 dimerization is critical for AR-V7 nuclear translocation and inhibition of AR-V7 acetylation impairs AR-V7 nuclear translocation. 2. Determine the clinical significance of ARD1 levels in CRPC progression and drug resistance. We will first determine and compare ARD1 levels in CTCs isolated from 11 mPCa and 50 mCRPC patients without any drug treatment to determine if a high level of ARD1 is correlated with mCRPC progression. Then, we will compare ARD1 levels in 50 CRPC patients who responded and 50 who did not respond to therapy with Abiraterone or Enzalutamide to determine if a high level of ARD1 is associated with drug resistance. Successful completion of this study will greatly increase our knowledge of the mechanistic roles and clinical significance of ARD1 in CRPC and nominate ARD1 as a novel biomarker or a therapeutic target for better management and treatment of CRPC patients.